Drive circuit for a light-emitting diode light source

ABSTRACT

A controllable lighting device may utilize a controllable impedance circuit to conduct a load current through an LED light source. The controllable impedance circuit may be coupled in series with a first switching device, which may be rendered conductive and non-conductive via a pulse-width modulated signal to adjust an average magnitude of the load current. The controllable lighting device may further comprise a control loop circuit that includes a second switching device. The second switching device may be rendered conductive and non-conductive in coordination with the first switching device to control when a feedback signal is provided to the control loop circuit and used to control the LED light source. The control loop circuit may be characterized by a time constant that is significantly greater than an operating period of the load current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional U.S. PatentApplication No. 62/725,467, filed Aug. 31, 2018, the disclosure of whichis incorporated herein by reference in its entirety.

BACKGROUND

Light-emitting diode (LED) light sources (e.g., LED light engines) arereplacing conventional incandescent, fluorescent, and halogen lamps as aprimary form of lighting devices. LED light sources may comprise aplurality of light-emitting diodes mounted on a single structure andprovided in a suitable housing. LED light sources may be more efficientand provide longer operational lives as compared to incandescent,fluorescent, and halogen lamps. An LED driver control device (e.g., anLED driver) may be coupled between a power source, such as analternating-current (AC) power source or a direct-current (DC) powersource, and an LED light source for regulating the power supplied to theLED light source. For example, the LED driver may regulate the voltageprovided to the LED light source, the current supplied to the LED lightsource, or both the current and voltage.

Different control techniques may be employed to drive LED light sourcesincluding, for example, a current load control technique and a voltageload control technique. An LED light source driven by the current loadcontrol technique may be characterized by a rated current (e.g.,approximately 350 milliamps) to which the magnitude (e.g., peak oraverage magnitude) of the current through the LED light source may beregulated to ensure that the LED light source is illuminated to theappropriate intensity and/or color. An LED light source driven by thevoltage load control technique may be characterized by a rated voltage(e.g., approximately 15 volts) to which the voltage across the LED lightsource may be regulated to ensure proper operation of the LED lightsource. If an LED light source rated for the voltage load controltechnique includes multiple parallel strings of LEDs, a current balanceregulation element may be used to ensure that the parallel strings havethe same impedance so that the same current is drawn in each of theparallel strings.

The light output of an LED light source may be dimmed. Methods fordimming an LED light source may include, for example, a pulse-widthmodulation (PWM) technique and a constant current reduction (CCR)technique. In pulse-width modulation dimming, a pulsed signal with avarying duty cycle may be supplied to the LED light source. For example,if the LED light source is being controlled using a current load controltechnique, the peak current supplied to the LED light source may be keptconstant during an on-time of the duty cycle of the pulsed signal. Theduty cycle of the pulsed signal may be varied, however, to vary theaverage current supplied to the LED light source, thereby changing theintensity of the light output of the LED light source. As anotherexample, if the LED light source is being controlled using a voltageload control technique, the voltage supplied to the LED light source maybe kept constant during the on-time of the duty cycle of the pulsedsignal. The duty cycle of the load voltage may be varied, however, toadjust the intensity of the light output. Constant current reductiondimming may be used if an LED light source is being controlled using thecurrent load control technique. In constant current reduction dimming,current may be continuously provided to the LED light source. The DCmagnitude of the current provided to the LED light source, however, maybe varied to adjust the intensity of the light output.

Examples of LED drivers are described in U.S. Pat. No. 8,492,987, issuedJul. 23, 2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODELIGHT SOURCE; U.S. Pat. No. 9,655,177, issued May 16, 2017, entitledFORWARD CONVERTER HAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT; and U.S.Pat. No. 9,247,608, issued Jan. 26, 2016, entitled LOAD CONTROL DEVICEFOR A LIGHT-EMITTING DIODE LIGHT SOURCE; the entire disclosures of whichare hereby incorporated by reference.

SUMMARY

Methods and apparatus are described herein for controlling an LED lightsource. A controllable impedance circuit may be coupled in series withthe LED light source and configured to conduct a load current throughthe LED light source. A first switching device may be connected inseries with the controllable impedance circuit while a feedback circuitis configured to generate a feedback signal indicative of a magnitude ofthe load current conducted through the LED light source. The feedbackcircuit may be coupled to a control loop circuit configured to generatea drive signal for controlling the controllable impedance circuit basedon the feedback signal. The control loop circuit may comprise a secondswitching device and/or a filter circuit. The second switching devicemay be capable of being rendered conductive and non-conductive tocontrol when the feedback signal is used to generate the drive signal(e.g., after the feedback signal is passed through the filter circuit).

A digital control circuit may control the control loop circuit to adjusta peak magnitude of the load current conducted through the LED lightsource toward a target magnitude. The digital control circuit may renderthe first switching device conductive and non-conductive via apulse-width modulated (PWM) signal and adjust a duty cycle of the PWMsignal to adjust an average magnitude of the load current. The digitalcontrol circuit may further render the second switching deviceconductive and non-conductive in coordination with the PWM signal. Forexample, the digital control circuit may be configured to render thesecond switching device conductive at the end of a first time periodafter the digital control circuit renders the first switching deviceconductive, and the digital control circuit may be further configured torender the second switching device non-conductive at the beginning of asecond time period before the digital control circuit renders the firstswitching device non-conductive.

The control loop circuit described herein may comprise an integratorcircuit. The control loop circuit may receive a target current controlsignal from the digital control circuit and generate the drive signal byintegrating the difference between the target current control signal andthe feedback signal via the integrator circuit. The control loop circuitmay be characterized by a time constant that is greater than a loadcurrent period of the load current conducted by the controllableimpedance circuit.

One or more of the components and/or functions described herein may beimplemented digitally. For example, sampling of the feedback signal maybe controlled by a digital control circuit and filtering operations maybe conducted using a digital low-pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a controllable electricaldevice, such as a controllable light source.

FIG. 2 is a simplified schematic diagram of a drive circuit, such as alight-emitting diode (LED) drive circuit, and a control loop circuit ofan electrical device, such as the controllable light source of FIG. 1.

FIG. 3 are example plots of the relationships between various operatingparameters of the controllable light source of FIG. 1 and a targetintensity of the controllable light source.

FIGS. 4A-4C are diagrams of simplified waveforms illustrating theoperation of the drive circuit and the control loop circuit of FIG. 2.

FIG. 5 is a simplified flow diagram of an example control procedure forcontrolling the control loop circuit of FIG. 2.

FIG. 6 is a simplified schematic diagram of a circuit that may be usedto realize the functionality of the drive circuit and the control loopcircuit shown in FIG. 2.

FIG. 7 is a simplified flow diagram of an example control procedure forcontrolling the circuit shown in FIG. 6.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a controllable electricaldevice, such as a controllable lighting device 100 (e.g., a controllablelight source). For example, the controllable lighting device 100 may bea lamp that comprise one or more light sources, such as light-emittingdiode (LED) light sources 102, 104 (e.g., LED light engines). The LEDlight sources 102, 104 may be controlled to adjust an intensity and/or acolor (e.g., a color temperature) of a cumulative light output of thecontrollable lighting device 100. Each LED light source 102, 104 isshown in FIG. 1 as a plurality of LEDs connected in series but maycomprise a single LED or a plurality of LEDs connected in parallel or asuitable combination thereof, depending on the particular lightingsystem. In addition, each LED light source 102, 104 may comprise one ormore organic light-emitting diodes (OLEDs). The controllable lightingdevice 100 may include a plurality of different LED light sources, whichmay be rated at different magnitudes of load current and voltage. Whilenot shown in FIG. 1, the controllable lighting device 100 may comprise ahousing (e.g., a translucent housing) in which the LED light sources arelocated and through which the LED light sources may shine. For example,the controllable lighting device 100 may be capable of providingwarm-dimming such that the color temperature of the cumulative lightoutput shifts towards a warm-white color temperature as the intensity ofthe cumulative light output is decreased. For example, the first LEDlight source 102 may comprise a white LED light source and the secondLED light source 104 may comprise a warm-white (e.g., red) LED lightsource, and the first LED light source 102 may have a higher powerrating than the second LED light source 104.

The controllable lighting device 100 may be a screw-in LED lampconfigured to be screwed into a standard Edison socket. The controllablelight device 100 may comprise a screw-in base that includes a hotconnection H and a neutral connection N for receiving analternating-current (AC) voltage V_(AC) from an AC power source (notshown). The hot connection H and the neutral connection N may also beconfigured to receive a direct-current (DC) voltage from a DC powersource. The controllable lighting device 100 may comprise aradio-frequency interference (RFI) filter and rectifier circuit 110,which may receive the AC voltage V_(AC). The RFI filter and rectifiercircuit 110 may operate to minimize the noise provided on the AC powersource and to generate a rectified voltage V_(RECT).

The controllable lighting device 100 may comprise a power convertercircuit 120, such as a flyback converter, which may receive therectified voltage V_(RECT) and generate a variable direct-current (DC)bus voltage V_(BUS) across a bus capacitor C_(BUS). The power convertercircuit 120 may comprise other types of power converter circuits, suchas, for example, a boost converter, a buck converter, a buck-boostconverter, a single-ended primary-inductance converter (SEPIC), a Ćukconverter, or any other suitable power converter circuit for generatingan appropriate bus voltage. The power converter circuit 120 may provideelectrical isolation between the AC power source and the LED lightsource 102, 104 and may operate as a power factor correction (PFC)circuit to adjust the power factor of the controllable lighting device100 towards a power factor of one.

As shown in FIG. 1, the flyback converter 120 may comprise a flybacktransformer 122, a field-effect transistor (FET) Q123, a diode D124, aresistor R125, a resistor R126, a flyback control circuit 127, and/or afeedback resistor R128. The flyback transformer 122 may comprise aprimary winding and a secondary winding. The primary winding may becoupled in series with the FET Q123. Although illustrated as the FETQ123, any switching transistor or other suitable semiconductor switchmay be coupled in series with the primary winding of the flybacktransformer 122. The secondary winding of the flyback transformer 122may be coupled to the bus capacitor C_(BUS) via the diode D124. A busvoltage feedback signal V_(BUS-FB) may be generated, e.g., by a voltagedivider comprising the resistors R125, R126 coupled across the buscapacitor C_(BUS). The flyback control circuit 127 may receive the busvoltage feedback signal V_(BUS-FB) and a control signal representativeof the current through the FET Q123 from the feedback resistor R128,which may be coupled in series with the FET Q123. The flyback controlcircuit 127 may control the FET Q123 to selectively conduct currentthrough the flyback transformer 122 to generate the bus voltage V_(BUS).The flyback control circuit 127 may render the FET Q123 conductive andnon-conductive, for example, to control the magnitude of the bus voltageV_(BUS) towards a target bus voltage V_(BUS-TRGT) in response to the DCmagnitude of the bus voltage feedback signal V_(BUS-FB) and themagnitude of the current through the FET Q123.

The controllable lighting device 100 may comprise one or more loadregulation circuits, such as LED drive circuits 130, 140, forcontrolling power delivered to (e.g., the intensities of) the LED lightsources 102, 104, respectively. The LED drive circuits 130, 140 may eachreceive the bus voltage V_(BUS) and may adjust magnitudes of respectiveload currents I_(LOAD1), I_(LOAD2) conducted through the LED lightsources 102, 104 and/or magnitudes of respective load voltagesV_(LOAD1), V_(LOAD2) generated across the LED light sources. One or moreof the LED drive circuits 130, 140 may comprise a controllable-impedancecircuit, such as a linear regulator, for example, as described herein.One or more of the LED drive circuits 130, 140 may comprise a switchingregulator, such as a buck converter for example. Examples of variousembodiments of LED drive circuits are described in U.S. Pat. No.8,492,987, filed Jul. 23, 2013, and U.S. Pat. No. 9,253,829, issued Feb.2, 2016, both entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODELIGHT SOURCE, the entire disclosures of which are hereby incorporated byreference.

The controllable lighting device 100 may comprise a control circuit 150for controlling the LED drive circuits 130, 140 to control themagnitudes of the respective load currents I_(LOAD1), I_(LOAD2)conducted through the LED light sources 102, 104 to adjust therespective intensities of the LED light sources. The control circuit 150may be configured to turn both of the LED light sources 102, 104 on andoff to turn the controllable lighting device 100 on and off,respectively. The control circuit 150 may be configured to control therespective intensities of the LED light sources 102, 104 to control theintensity and/or the color (e.g., the color temperature) of thecumulative light emitted by the controllable lighting device 100. Thecontrol circuit 150 may be configured to adjust (e.g., dim) a presentintensity L_(PRES) of the cumulative light emitted by the controllablelighting device 100 towards a target intensity L_(TRGT), which may rangeacross a dimming range of the controllable light source, e.g., between alow-end intensity L_(LE) (e.g., a minimum intensity, such asapproximately 0.1%-1.0%) and a high-end intensity L_(HE) (e.g., amaximum intensity, such as approximately 100%). The control circuit 150may be configured to adjust a present color temperature T_(PRES) of thecumulative light emitted by the controllable lighting device 100 towardsa target color temperature T_(TRGT), which may range between acool-white color temperature (e.g., approximately 3100-4500 K) and awarm-white color temperature (e.g., approximately 2000-3000 K).

The control circuit 150 may comprise a digital control circuit 152, suchas, for example, a microprocessor, a microcontroller, a programmablelogic device (PLD), an application specific integrated circuit (ASIC), afield-programmable gate array (FPGA), or any other suitable processingdevice or controller. The control circuit 150 may comprise a memory (notshown) configured to store operational characteristics of thecontrollable lighting device 100 (e.g., the target intensity L_(TRGT),the target color temperature T_(TRGT), the low-end intensity L_(LE), thehigh-end intensity L_(HE), etc.). The memory may be implemented as anexternal integrated circuit (IC) or as an internal circuit of thedigital control circuit 152. The controllable lighting device 100 maycomprise a power supply 160 that may be coupled to a winding 162 of theflyback transformer 122 of the power converter circuit 120 and may beconfigured to generate a supply voltage V_(CC) for powering the digitalcontrol circuit 152 and other low-voltage circuitry of the controllablelighting device.

The control circuit 150 may also comprise control loop circuits (e.g.,analog control loop circuits) 154, 156 for controlling the LED drivecircuits 130, 140, respectively. The LED drive circuits 130, 140 maycomprise respective regulation devices (e.g., regulation field-effecttransistors (FET) Q132, Q142) coupled (e.g., in series) with the LEDlight sources 102, 104, respectively, for conducting the load currentsI_(LOAD1), I_(LOAD2). Each regulation FET Q132, Q142 may comprise anytype of suitable power semiconductor switch, such as, for example, abipolar junction transistor (BJT), and/or an insulated-gate bipolartransistor (IGBT). The control loop circuits 154, 156 may generaterespective drive signals V_(DR1), V_(DR2) that may be received by gatesof the regulation FETs Q132, Q142 for controlling the regulation FETs inthe linear region to provide controllable impedances in series with theLED light sources 102, 104, respectively (e.g., to operate theregulation FETs Q132, Q142 as linear regulators). When the regulationFETs Q132, Q142 are conducting the respective load currents I_(LOAD1),I_(LOAD2), respective regulator voltages V_(R1), V_(R2) may be developedacross the regulation FETs Q132, Q142.

The regulation FETs Q132, Q142 may be coupled (e.g., in series) withrespective feedback circuits (e.g., current feedback (CFB) circuits)134, 144. The current feedback circuits 134, 144 may be coupled to thecontrol loop circuits 154, 156 of the control circuit 150 and maygenerate respective current feedback signals V_(FB1), V_(FB2), which maybe received by the control loop circuits 154, 156. The control loopcircuits 154, 156 may be configured to adjust the magnitudes (e.g., DCmagnitudes) of the drive signals V_(DR1), V_(DR2) provided to the gatesof the regulation FETs Q132, Q142 in response to the magnitudes of thecurrent feedback signals V_(FB1), V_(FB2), respectively. The digitalcontrol circuit 152 may generate respective target-current controlsignals V_(TRGT1), V_(TRGT2), which may also be received by the controlloop circuits 154, 156. The control loop circuits 154, 156 may beconfigured to adjust the magnitudes of the drive signals V_(DR1),V_(DR2) provided to the gates of the regulation FETs Q132, Q142 tocontrol the magnitudes of the load current I_(LOAD1), I_(LOAD2) towardsrespective target current I_(TRGT1), I_(TRGT2) that are set by thetarget-current control signals V_(TRGT1), V_(TRGT2).

The LED drive circuits 130, 140 may further comprise dimming devices(e.g., dimming FETs Q136, Q146 or another type of semiconductorswitches) coupled (e.g., in series) with the regulation FETs Q132, Q142and the current feedback circuits 134, 144, respectively. The digitalcontrol circuit 152 may generate respective dimming control signalsV_(DIM1), V_(DIM2) that may be received by gates of the respectivedimming FETs Q136, Q146 for rendering the dimming FETs conductive andnon-conductive to adjust average magnitudes of the load currentsI_(LOAD1), I_(LOAD2), respectively. For example, the digital controlcircuit 152 may be configured to pulse-width modulate (PWM) the loadcurrents I_(LOAD1), I_(LOAD2) by generating the dimming control signalsV_(DIM1), V_(DIM2) as pulse-width modulated (PWM) signals at a dimmingfrequency f_(DIM). The digital control circuit 152 may be configured toadjust respective duty cycles DC₁, DC₂ of the dimming control signalsV_(DIM1), V_(DIM2) to adjust the average magnitudes of the load currentsI_(LOAD1), I_(LOAD2), respectively. When the digital control circuit 152is pulse-width modulating the dimming control signals V_(DIM1),V_(DIM2), the load currents I_(LOAD1), I_(LOAD2) may be characterized bya load-current frequency f_(LOAD) (e.g., which is approximately equal tothe dimming frequency f_(DIM) of the dimming control signals V_(DIM1),V_(DIM2)) and a corresponding load-current period T_(LOAD). Theload-current frequency f_(LOAD) may be high enough to prevent flickeringthat may be visible to the human eye.

The dimming FETs Q136, Q146 may be coupled between the respectivecurrent feedback circuits 134, 144 and circuit common. The digitalcontrol circuit 152 may be configured to control when the control loopcircuits 154, 156 are responsive to the respective current feedbacksignals V_(FB1), V_(FB2) for adjusting the magnitudes of the drivesignals V_(DR1), V_(DR2). The digital control circuit 152 may beconfigured to render the control circuit circuits 154, 156 responsiveand non-responsive to the respective current feedback signals V_(FB1),V_(FB2) in coordination with the respective dimming control signalsV_(DIM1), V_(DIM2). For example, the digital control circuit 152 may beconfigured to enable the control loop circuits 154, 156 to be responsiveto the respective current feedback signals V_(FB1), V_(FB2) duringfeedback windows TWIN that may be approximately the same length as orslightly shorter than the time periods when the dimming FETs Q136, Q146are rendered conductive. The digital control circuit 152 may beconfigured to render the control loop circuits 154, 156 responsive tothe respective current feedback signals V_(FB1), V_(FB2) atapproximately the same time or slightly after the dimming FETs Q136,Q146 are rendered conductive. The digital control circuit 152 may beconfigured to render the control loop circuits 154, 156 non-responsiveto the respective current feedback signals V_(FB1), V_(FB2) atapproximately the same time or slightly before the dimming FETs Q136,Q146 are rendered non-conductive. To control the operation of therespective control loop circuits 154, 156, the digital control circuit152 may generate respective feedback window control signals V_(WIN1),V_(WIN2) that may be received by the control loop circuits for enablingand disabling when the control loop circuits are responsive to therespective current feedback signals V_(FB1), V_(FB2). As a result, eachcontrol loop circuit 154, 156 may be responsive to a peak magnitudeI_(PK) of the respective current feedback signal V_(FB1), V_(FB2) (e.g.,when the dimming FETs Q136, Q146 are conductive).

The techniques described herein may help prevent erroneous operation ofthe controllable lighting device 100 in various situations. For example,since the dimming FETs Q136, Q146 may be coupled between the respectivecurrent feedback circuits 134, 144 and the circuit common, themagnitudes of the current feedback signals V_(FB1), V_(FB2) may bepulled up towards the bus voltage V_(BUS) when the dimming FETs Q136,Q146 are non-conductive, which may cause the control loop circuits 154,156 to incorrectly drive the regulation FETs Q132, Q142. By configuringthe digital control circuit 152 to control (e.g., at least with respectto timing) when the control loop circuits 154, 156 are responsive to therespective current feedback signals V_(FB1), V_(FB2) for adjusting themagnitudes of the drive signals V_(DR1), V_(DR2), erroneous generationof the drive signals V_(DR1), V_(DR2) may be avoided.

The controllable lighting device 100 may comprise a bus adjustmentcircuit 170 for controlling the magnitude of the bus voltage V_(BUS)(e.g., to make sure that the regulation FETs Q132, Q142 do not dissipatetoo much power). For example, the bus adjustment circuit 170 may becoupled to the junction of the first regulation FET Q132 and the firstLED light source 102, and may be responsive to the first regulatorvoltages V_(R1) across the first regulation FET Q132. The bus adjustmentcircuit 170 may be coupled to the junction of the resistors R125, R126for adjusting the magnitude of the bus voltage feedback signalV_(BUS-FB) to cause the flyback control circuit 127 to adjust themagnitude of the bus voltage V_(BUS). For example, the bus adjustmentcircuit 170 may adjust the magnitude of the bus voltage V_(BUS) tocontrol the magnitude of the first regulator voltage V_(R1) to be lessthan a maximum regulator voltage threshold V_(R-MAX) (e.g.,approximately 0.6 volts), for example, to prevent the power dissipatedin regulation FETs Q132, Q142 from becoming too large. In examples(e.g., as shown in FIG. 1), the bus adjustment circuit 170 may be onlycoupled to the first regulation transistor Q132. Since the first LEDlight source 102 may have a higher power rating than the second LEDlight source 104 (as previously mentioned), adjustments of the magnitudeof the bus voltage V_(BUS) in response to the magnitude of the firstregulator voltage V_(R1) to make sure that the first regulation FET Q132does not dissipate too much power may also ensure that the secondregulation FET Q142 does not dissipate too much power.

The controllable lighting device 100 may comprise a communicationcircuit 180 coupled to the digital control circuit 152. Thecommunication circuit 180 may comprise a wireless communication circuit,such as, for example, a radio-frequency (RF) transceiver coupled to anantenna 182 for transmitting and/or receiving RF signals. The wirelesscommunication circuit may be an RF transmitter for transmitting RFsignals, an RF receiver for receiving RF signals, or an infrared (IR)transmitter and/or receiver for transmitting and/or receiving IRsignals. The communication circuit 180 may be coupled to the hotconnection H and the neutral connection N of the controllable lightingdevice 100 for transmitting a control signal via the electrical wiringusing, for example, a power-line carrier (PLC) communication technique.The digital control circuit 152 may be configured to determine thetarget intensity L_(TRGT) for the controllable lighting device 100 inresponse to messages (e.g., digital messages) received via thecommunication circuit 180. The digital control circuit 152 may beconfigured to determine respective target intensities L_(TRGT1),L_(TRGT2) for the LED light sources 102, 104 in response to thedetermined target intensity L_(TRGT) for the controllable lightingdevice 100.

When the target intensity L_(TRGT1), L_(TRGT2) of at least one of theLED light sources 102, 104 is greater than or equal to a transitionintensity L_(TRAN), the digital control circuit 152 may be configured torender the respective dimming FET Q136, Q146 conductive (e.g.,conductive at all times) and adjust the intensity of the respective LEDlight source using a constant current reduction (CCR) dimming technique.Using the CCR dimming technique, the digital control circuit 152 mayadjust the respective target-current control signal V_(TRGT1), V_(TRGT2)to cause the respective control loop circuit 154, 156 to adjust theaverage magnitude of the load current I_(LOAD1), I_(LOAD2) towards therespective target current I_(TRGT1), I_(TRGT2). The target currentsI_(TRGT1), I_(TRGT2) may each range between a maximum current I_(MAX)(e.g., at the high-end intensity L_(HE)) and a minimum current I_(MIN)(e.g., at the transition intensity L_(TRAN)).

When the target intensity L_(TRGT1), L_(TRGT2) of at least one of theLED light sources 102, 104 is less than the transition intensityL_(TRAN), the digital control circuit 152 may be configured to controlthe respective dimming FET Q136, Q146 to adjust the intensity of therespective LED light source using a pulse-width modulation (PWM) dimmingtechnique. For example, the digital control circuit 152 may beconfigured to control the respective target-current control signalV_(TRGT1), V_(TRGT2) to maintain the respective target currentI_(TRGT1), I_(TRGT2) constant. Using the PWM dimming technique, thedigital control circuit 152 may adjust the duty cycle DC₁, DC₂ of therespective dimming control signal V_(DIM1), V_(DIM2) to adjust theaverage magnitude of the load current I_(LOAD1), I_(LOAD2) below theminimum current I_(MIN). For example, the digital control circuit 152may adjust the duty cycle DC₁, DC₂ of each of the dimming controlsignals V_(DIM1), V_(DIM2) as a function of the respective targetintensity L_(TRGT1), L_(TRGT2). For example, the digital control circuit152 may linearly decrease the duty cycle DC₁, DC₂ as the respectivetarget intensity L_(TRGT1), L_(TRGT2) decreases, and vice versa. Eachcontrol loop circuit 154, 156 may continue to regulate the peakmagnitudes I_(PK) of the load current I_(LOAD1), I_(LOAD2) towards thetarget current I_(TRGT1), I_(TRGT2) when the respective dimming FETQ136, Q146 is conductive. Each control loop circuit 154, 156 may becharacterized by a time constant that is much greater than theload-current period T_(LOAD) of the respective load current I_(LOAD1),I_(LOAD2), for example, to help avoid changes in the magnitudes of therespective drive signals V_(DR1), V_(DR2) when the dimming FETs Q136,Q146 are non-conductive. The time constant may be associated with one ormore integrator circuit and/or RC filter circuits comprised in thecontrol loop circuit 154, 156, for example. The value of the timeconstant may be determined by the electrical properties (e.g.,capacitance and/or resistance) of one or more components comprised inthe control loop circuit 154, 156.

FIG. 2 is a simplified schematic diagram of an LED drive circuit 210(e.g., one of the LED drive circuits 130, 140) and a control loopcircuit 220 (e.g., one of the control loop circuits 154, 156) of anelectrical device 200, such as an LED driver or a controllable lightsource (e.g., the controllable lighting device 100). The LED drivecircuit 210 may be coupled in series with an LED light source 202 (e.g.,one of the LED light sources 102, 104) for conducting a load currentI_(LOAD) through the LED light source. The control loop circuit 220 maygenerate a drive signal V_(DR) for controlling the LED drive circuit 210to adjust a magnitude of the load current I_(LOAD) through the LED lightsource. The LED driver 100 may also comprise a digital control circuit252 (e.g., the digital control circuit 152) for generating a PWM controlsignal (e.g., a target-current control signal V_(TRGT-PWM)) that may bereceived by the control loop circuit 220 for setting a target currentI_(TRGT) for the load current I_(LOAD). The digital control circuit 252may be configured to adjust the intensity of the LED light source 202towards a target intensity L_(TRGT) that may range between a minimumintensity L_(MIN) (e.g., approximately 0.1%-1.0%) and a maximumintensity L_(MAX) (e.g., approximately 100%). The minimum intensityL_(MIN) may be approximately the lowest intensity at which the digitalcontrol circuit 252 may control the LED light source 202 under steadystate conditions (e.g., when the target intensity L_(TRGT) is being heldconstant).

The LED drive circuit 210 may comprise a regulation device such as aregulation FET Q212 coupled in series with the LED light source 202. Theregulation FET Q212 may comprise any type of suitable powersemiconductor switch, such as, for example, a bipolar junctiontransistor (BJT), and/or an insulated-gate bipolar transistor (IGBT).When the regulation FET Q212 is conductive, a regulator voltage V_(R)may be developed across the regulation FET. The LED drive circuit 210may comprise a current feedback circuit (e.g., a current feedbackresistor 8214) coupled in series with the regulation FET Q212 forgenerating a current feedback signal V_(FB) that may have a DC magnituderepresentative of the magnitude of the load current I_(LOAD). The LEDdrive circuit 210 may comprise a dimming device (e.g., such as a dimmingFET Q216 or another type of semiconductor switch) coupled between thecurrent feedback resistor 8214 and circuit common. The digital controlcircuit 252 may generate a dimming control signal V_(DIM) that may bereceived by a gate of the dimming FET Q216. The dimming FET Q216 may berendered conductive and non-conductive in response to the dimmingcontrol signal V_(DIM) for adjusting an average magnitude of the loadcurrent I_(LOAD).

The control loop circuit 220 may receive the current feedback signalV_(FB) generated by the current feedback resistor 8214 and/or the PWMtarget-current control signal V_(TRGT-PWM) generated by the digitalcontrol circuit 252. The current feedback signal V_(FB) may be receivedby a controllable switch 222 comprised in the control loop circuit 220.The controllable switch 222 may be rendered conductive andnon-conductive in response to a feedback window control signal V_(WIN)(e.g., a switch control signal) generated by the digital control circuit252. The controllable switch 222 may be coupled to a filter circuit,which may comprise a capacitor C224 and a resistor 8225. When thecontrollable switch 222 is conductive, the capacitor C224 (e.g., havinga capacitance of approximately 1.0 μF) may charge to approximately apeak magnitude I_(PK) of the current feedback signal V_(FB) through theresistor R225 (e.g., having a resistance of approximately 100) forgenerating a peak-current feedback signal V_(FB-PK) across thecapacitor.

The control loop circuit 220 may comprise an operational amplifier U226comprising an inverting input coupled to receive the current feedbacksignal V_(FB) through a resistor R228. The control loop circuit 220 maycomprise a filter circuit (e.g., a low-pass RC filter circuit) includinga resistor R230 (e.g., having a resistance of approximately 1 kΩ) and acapacitor C232 (e.g., having a capacitance of approximately 0.1 μF). ThePWM target-current control signal V_(TRGT-PWM) may be received by theresistor R230, such that a DC target-current control signal V_(TRGT-DC)is generated at the junction of the resistor R230 and the capacitor C232and has a DC magnitude representative of the target current I_(TRGT) forthe load current I_(LOAD). The DC target-current control signalV_(TRGT-DC) may be coupled to a non-inverting input of the operationalamplifier U226. For example, the digital control circuit 252 maygenerate the PWM target-current control signal V_(TRGT-PWM) as apulse-width modulated signal having a duty cycle DC_(TRGT)representative of the target current I_(TRGT) for the load currentI_(LOAD). In addition, the digital control circuit 252 may comprise adigital-to-analog converter (DAC) for generating the DC target-currentcontrol signal V_(TRGT-DC) that may be directly coupled to thenon-inverting input of the operational amplifier U226 (e.g., withoutrequiring the resistor R230 and the capacitor C232).

The control loop circuit 220 may comprise a capacitor C234 coupledbetween the inverting input and an output of the operational amplifierU226, such that the control loop circuit 220 may be configured tointegrate the error between the peak-current feedback signal V_(FB-PK)and the DC target-current control signal V_(TRGT-DC). The control loopcircuit 220 may generate the drive signal V_(DR) that may be received bya gate of the regulation FET Q212 for controlling the regulation FET inthe linear region to provide a controllable impedance in series with theLED light source 202 (e.g., the regulation FET may be operated as alinear regulator). The output of the operational amplifier U226 may becoupled to the gate of the regulation FET Q212 through another filtercircuit (e.g., a low-pass RC filter circuit) including a resistor R236(e.g., having a resistance of approximately 1 kΩ) and a capacitor C238(e.g., having a capacitance of approximately 0.1 μF).

The digital control circuit 252 may control the dimming control signalV_(DIM) to render the dimming FET Q216 conductive and non-conductive toadjust average magnitude of the load current I_(LOAD). For example, thedigital control circuit 252 may be configured to pulse-width modulate(PWM) the load current I_(LOAD) by generating the dimming control signalV_(DIM) as a pulse-width modulated (PWM) signal at a dimming frequencyf_(DIM). The digital control circuit 252 may be configured to adjust aduty cycle DC_(DIM) of the dimming control signal V_(DIM) to adjust theaverage magnitude of the load current I_(LOAD). When the digital controlcircuit 252 is pulse-width modulating the dimming control signalV_(DIM), the load current I_(LOAD) may be characterized by aload-current frequency f_(LOAD) that is approximately equal to thedimming frequency of the dimming control signal V_(DIM). Theload-current frequency f_(LOAD) may be high enough to prevent flickeringin the LED light source 202 that may be visible to the human eye.

The digital control circuit 252 may be configured to render thecontrollable switch 222 conductive and non-conductive in coordinationwith the dimming control signal V_(DIM). For example, the digitalcontrol circuit 252 may be configured to render the controllable switch222 conductive at approximately the same time or slightly after thedigital control circuit renders the dimming FET Q216 conductive. Thedigital control circuit 252 may be configured to render the controllableswitch 222 non-conductive at approximately the same time or slightlybefore the digital control circuit renders the dimming FET Q216non-conductive. This way, the magnitude of the peak-current feedbacksignal V_(FB-PK) may be representative of a peak magnitude I_(PK) of theload current I_(LOAD), which may prevent erroneous operation of thecontrol circuitry in various situations. For example, since the dimmingFET Q216 may be coupled between the current feedback resistor R214 andcircuit common, the magnitude of the current feedback signals V_(FB) maybe pulled up towards the bus voltage V_(BUS) when the dimming FET Q216is non-conductive. This may cause the control loop circuit 220 toincorrectly drive the regulation FETs Q212. By configuring the digitalcontrol circuit 252 to control when the controllable switch 222 isrendered conductive, erroneous generation of the drive signal V_(DR) maybe avoided.

The digital control circuit 252 may control the duty cycle DC_(TRGT) ofthe PWM target-current control signal V_(TRGT-PWM), the duty cycleDC_(DIM) of the dimming control signal V_(DIM), and/or the dimmingfrequency f_(DIM) of the dimming control signal V_(DIM) as a function ofthe target intensity L_(TRGT). FIG. 3 shows example plots of therelationship between the peak current I_(PK) of the load currentI_(LOAD) and the target intensity L_(TRGT), the relationship between theduty cycle DC_(DIM) of the dimming control signal V_(DIM) and the targetintensity L_(TRGT), and the relationship between the dimming frequencyf_(DIM) of the dimming control signal V_(DIM) and the target intensityL_(TRGT).

When the target intensity L_(TRGT) of the LED light source 202 isgreater than or equal to a transition intensity L_(TRAN), the digitalcontrol circuit 252 may be configured to render the dimming FET Q216conductive (e.g., conductive at approximately all times) and adjust thepeak magnitude I_(PK) of the load current I_(LOAD) to adjust theintensity of the LED light source (e.g., using a constant currentreduction (CCR) dimming technique). For example, the digital controlcircuit 252 may adjust the duty cycle DC_(TRGT) of the PWMtarget-current control signal V_(TRGT-PWM) to cause the control loopcircuit 220 to adjust the peak magnitude I_(PK) of the load currentI_(LOAD) towards the target current I_(TRGT), which may range between amaximum current I_(MAX) and a minimum current I_(MIN). When the targetintensity L_(TRGT) of the LED light source 202 is greater than or equalto the transition intensity L_(TRAN), the duty cycle DC_(DIM) of thedimming control signal V_(DIM) may be held constant at a maximum dutycycle DC_(MAX). The maximum duty cycle DC_(MAX) may be less than 100%(e.g., as shown in FIG. 3), such that the digital control circuit 252may pulse-width modulate the load current I_(LOAD). The maximum dutycycle DC_(MAX) may be equal to 100%, such that the dimming FET Q216 maybe conductive at all times when the target intensity L_(TRGT) of the LEDlight source 202 is greater than or equal to the transition intensityL_(TRAN).

When the target intensity L_(TRGT) of the LED light source 202 is lessthan the transition intensity L_(TRAN), the digital control circuit 252may be configured to control the dimming FET Q216 to adjust theintensity of the LED light source (e.g., using a pulse-width modulation(PWM) dimming technique). When using the PWM dimming technique, thedigital control circuit 252 may be configured to maintain the duty cycleDC_(TRGT) of the target-current control signal V_(TRGT-PWM) constant tomaintain the target current I_(TRGT) constant, and adjust the duty cycleDC_(DIM) of the dimming control signal V_(DIM) to adjust the magnitudeof the load current I_(LOAD). For example, the digital control circuit252 may adjust the duty cycle DC_(DIM) as a function of the targetintensity L_(TRGT) (e.g., linearly) as shown in FIG. 3. The control loopcircuit 220 may continue to regulate the peak magnitude I_(PK) of theload current I_(LOAD) towards the target current I_(TRGT) when thedimming FET Q216 is conductive. The control loop circuit 220 may becharacterized by a time constant that is much greater than theload-current period T_(LOAD) of the load current I_(LOAD), for example,to help avoid changes in the magnitude of the drive signal V_(DR) whenthe dimming FET Q216 is non-conductive.

The digital control circuit 252 may be configured to fade (e.g.,gradually adjust over a period of time) the target intensity L_(TRGT)(and thus the present intensity) of the LED light source 202. Thedigital control circuit 252 may be configured to fade the LED lightsource 202 from off to on by slowly increasing the present intensityL_(PRES) of the LED light source from a minimum fading intensityL_(FADE-MIN), which may be less than the minimum intensity L_(MIN)(e.g., such as approximately 0.02%), to the target intensity L_(TRGT).The digital control circuit 252 may be configured to fade the LED lightsource 202 from on to off by slowly decreasing the present intensityL_(PRES) of the LED light source from an initial intensity greater thanor equal to the minimum intensity L_(MIN) to the minimum fadingintensity L_(FADE-MIN) at which point the digital control circuit 252may turn off the LED light source. As shown in FIG. 3, when the targetintensity L_(TRGT) is less than the minimum intensity L_(MIN), thedigital control circuit 252 may adjust the dimming frequency f_(DIM) ofthe dimming control signal V_(DIM) with respect to the target currentI_(TRGT) (e.g., while holding the duty cycle DC_(TRGT) of thetarget-current control signal V_(TRGT-PWM) and the duty cycle DC_(DIM)of the dimming control signal V_(DIM) constant).

FIGS. 4A-4C show waveforms that illustrate the operation of the LEDdrive circuit 210 and the control loop circuit 220 of FIG. 2. In theexample shown in FIG. 4A, the target intensity L_(TRGT) may be equal toand/or close to the maximum intensity L_(MAX). The peak current I_(PK)of the load current I_(LOAD) may be controlled to the maximum currentI_(MAX). The duty cycle DC_(DIM) of the dimming control signal V_(DIM)may be controlled to the maximum duty cycle DC_(MAX) (e.g., at 99%)resulting in prolonged on time T_(ON) of the dimming control signal. Thedigital control signal 252 may drive the window control signal V_(WIN)high towards the supply voltage V_(CC) after a first offset time periodT_(OFFSET1) from when the dimming control signal V_(DIM) is driven high.The digital control signal 252 may drive the window control signal WINlow towards circuit common at a time that is a second offset time periodT_(OFFSET2) before when the dimming control signal V_(DIM) is drivenlow. The peak current feedback signal V_(FB-PK) may have a magnitudethat is dependent upon (e.g., representative of) the peak magnitudeI_(PK) of the load current I_(LOAD) (e.g., the maximum current I_(MAX)).The drive signal V_(DR) provided to the gate of the regulationtransistor Q212 may be at a first magnitude V_(DR1).

In the example shown in FIG. 4B, the target intensity L_(TRGT) may beequal to approximately the transition intensity L_(TRAN). The peakcurrent I_(PK) of the load current I_(LOAD) may be controlled toapproximately the minimum current I_(MIN). The duty cycle DC_(DIM) ofthe dimming control signal V_(DIM) may still be controlled to themaximum duty cycle DC_(MAX) resulting in a similar on time T_(ON) of thedimming control signal as shown in FIG. 4A. The digital control signal252 may drive the window control signal WIN high towards the supplyvoltage V_(CC) after a first offset time period T_(OFFSET1) from whenthe dimming control signal V_(DIM) is driven high. The digital controlsignal 252 may drive the window control signal WIN low towards circuitcommon at a time that is a second offset time period T_(OFFSET2) beforewhen the dimming control signal V_(DIM) is driven low. The peak currentfeedback signal V_(FB-PK) may have a magnitude that is dependent upon(e.g., representative of) the peak magnitude INK of the load currentI_(LOAD) (e.g., the minimum current I_(MIN)). The drive signal V_(DR)provided to the gate of the regulation transistor Q212 may be at asecond magnitude V_(DR2).

In the example shown in FIG. 4C, the target intensity L_(TRGT) may beless than the transition intensity L_(TRAN) and greater than the minimumintensity L_(MIN). As in FIG. 4B, the peak current INK of the loadcurrent I_(LOAD) may be controlled to be approximately the minimumcurrent I_(MIN). The duty cycle DC_(DIM) of the dimming control signalV_(DIM) may be controlled to be less than the maximum duty cycleDC_(MAX) resulting in smaller on time T_(ON) of the dimming controlsignal than shown in FIGS. 4A and 4B. The digital control signal 252 maydrive the window control signal V_(WIN) high towards the supply voltageV_(CC) after a first offset time period T_(OFFSET1) from when thedimming control signal V_(DIM) is driven high. The digital controlsignal 252 may drive the window control signal WIN low towards circuitcommon at a time that is a second offset time period T_(OFFSET2) beforewhen the dimming control signal V_(DIM) is driven low. The peak currentfeedback signal V_(FB-PK) may have a magnitude that is dependent uponthe peak magnitude INK of the load current I_(LOAD) (e.g., the minimumcurrent I_(MIN)). The drive signal V_(DR) provided to the gate of theregulation transistor Q212 may be at approximately the second magnitudeV_(DR2) (e.g., as in FIG. 4B).

FIG. 5 is a simplified flow diagram of an example control procedure 500for controlling a control loop circuit as described herein (e.g., thecontrol loop circuit 220 of FIG. 2). The control procedure 500 may beexecuted by the digital control circuit 252 at step 510, for example,periodically and/or in response to a change of the target currentI_(TRGT) for the light source 202. At 512, the digital control circuitmay determine the on time T_(ON) of the dimming control signal V_(DIM),for example, based on the present duty cycle of the dimming controlsignal. A timer may be started at 514 and the dimming FET Q216 may berendered conductive at 516, for example by driving the dimming controlsignal V_(DIM) high (e.g., at approximately the same time that the timeris started). The value of the timer may be compared (e.g., periodically)to a first offset time period T_(OFFSET1) at 518. Once the timer valuereaches the first offset time period T_(OFFSET1), the digital controlcircuit 252 may render the controllable switch 222 conductive at 520,for example by driving the feedback window control signal V_(WIN) high.The digital control circuit 252 may then continue to check (e.g.,periodically) the value of the timer at 522 against a value that isequal to the difference between the on time T_(ON) and a second offsettime period T_(OFFSET2) (e.g., T_(ON)−T_(OFFSET2)). Once the timer valuereaches the difference between the on time T_(ON) and a second offsettime period T_(OFFSET2), the digital control circuit 252 may render thecontrollable switch 222 non-conductive at 524, for example by drivingfeedback window control signal V_(WIN) low. Subsequently, the digitalcontrol circuit 252 may continue to monitor the value of the timer at526 until that value reaches the on time T_(ON). At that point, thedigital control circuit 252 may render the dimming FET Q216non-conductive at 528, for example by driving the dimming control signalV_(DIM) high, and the control procedure 500 may exit.

Part or the entirety of the functionality of the control loop circuit220 may be implemented in a digital control circuit (e.g., the digitalcontrol circuit 252 or another digital control circuit of the controldevice 200). FIG. 6 is a simplified schematic diagram of a circuit 600that may be used to realize the functionality of the LED drive circuit210 and/or the control loop circuit 220 shown in FIG. 2. The circuit 600may comprise an LED drive circuit 610, which may be implemented andconfigured in a similar manner as the LED drive circuit 210. Forexample, the LED drive circuit 610 may comprise a regulation device suchas a regulation FET Q612 (e.g., similar to the regulation FET Q212). TheLED drive circuit 610 may comprise a current feedback circuit (e.g., acurrent feedback resistor 614, which may be similar to the currentfeedback resistor R214). The LED drive circuit 610 may further comprisea dimming device such as a dimming FET Q616 (e.g., similar to thedimming FET Q216) coupled between the current feedback resistor 614 andcircuit common. The digital control circuit 652 may generate a dimmingcontrol signal V_(DIM) that may be received by a gate of the dimming FETQ616. The dimming FET Q616 may be rendered conductive and non-conductivein response to the dimming control signal V_(DIM) for adjusting anaverage magnitude of the load current I_(LOAD) conducted through an LEDlight source 602.

The digital control circuit 652 may sample a current feedback signalV_(FB) generated via the current feedback resistor 614 during a timewindow in order to derive an average of the feedback signal that may berepresentative of a peak magnitude I_(PK) of the load current I_(LOAD).The digital control circuit 652 may control the time window incoordination with the dimming control signal V_(DIM). For example, thedigital control circuit 652 may control the time window to start atapproximately the same time or slightly after (e.g., an offset timeperiod after) the time when the digital control circuit renders thedimming FET Q616 conductive. The digital control circuit 652 may controlthe time window to end at approximately the same time or slightly before(e.g., an offset time period before) the time when the digital controlcircuit renders the dimming FET Q616 non-conductive. The derivedfeedback signal may be filtered (e.g., via a digital low pass filter)and used to generate a drive signal V_(DR) that may be received by agate of the regulation FET Q612 for controlling the regulation FET inthe linear region to provide a controllable impedance in series with theLED light source 602 (e.g., to operate the regulation FET as a linearregulator).

The digital control circuit 652 may control the dimming control signalV_(DIM) to render the dimming FET Q616 conductive and non-conductive toadjust an average magnitude of the load current I_(LOAD). For example,the digital control circuit 652 may be configured to pulse-widthmodulate the load current I_(LOAD) by generating the dimming controlsignal V_(DIM) as a pulse-width modulated signal at a dimming frequencyf_(DIM). The digital control circuit 652 may be configured to adjust aduty cycle DC_(DIM) of the dimming control signal V_(DIM) to adjust theaverage magnitude of the load current I_(LOAD). When the digital controlcircuit 652 is pulse-width modulating the dimming control signalV_(DIM), the load current I_(LOAD) may be characterized by aload-current frequency f_(LOAD) that is approximately equal to thedimming frequency of the dimming control signal V_(DIM). Theload-current frequency f_(LOAD) may be high enough to prevent flickeringin the LED light source 602 that may be visible to the human eye. Thedigital control circuit 652 may be configured to maintain the magnitudeof the drive signal V_(DR) when the dimming FETs Q616 is non-conductive.

FIG. 7 is a simplified flow diagram of an example control procedure 700for controlling the circuit 600 shown in FIG. 6. The control procedure700 may be executed by the digital control circuit 652 at step 710, forexample, periodically and/or in response to a change of the targetcurrent I_(TRGT) for the light source 602. At 712, the digital controlcircuit 652 may determine the on time T_(ON) of the dimming controlsignal V_(DIM), for example, based on the present duty cycle of thedimming control signal. A timer may be started at 714 when the dimmingFET Q616 is rendered conductive at 716. The value of the timer may becompared (e.g., periodically compared) to a first offset time periodT_(OFFSET1) at 718. Once the timer value reaches the first offset timeperiod T_(OFFSET1) but is still less than the on time T_(ON) by at leasta second offset time period T_(OFFSET2) (e.g.,Timer<T_(ON)−T_(OFFSET2)), the digital control circuit 652 mayrepetitively sample the current feedback signal V_(FB) at 720 andcalculate an average of the samples at 722. At 724, the digital controlcircuit 652 may determine that the timer value has reachedT_(ON)−T_(OFFSET2) and may subsequently stop sampling the currentfeedback signal V_(FB) at 726. The digital control circuit 652 mayfurther determine, at 728, that the end of the on time T_(ON) has beenreached, at which point the digital control circuit 652 may render thedimming FET 616 non-conductive at 730 and may process the average valueof the current feedback signal V_(FB) to determine an appropriate levelfor the drive signal V_(DR) at 732. The determined level for the drivesignal V_(DR) may be filtered (e.g., using a digital low pass filter(LPF)) at 734. Based on the filtered level, the digital control circuit562 may generate a DC voltage at 736 (e.g., using a DAC or by generatinga PWM signal that may be filtered with an external RC filter) fordriving the regulation FET 612. The control procedure 700 may then exit.

Although described with reference to a controllable light source and/oran LED driver, one or more embodiments described herein may be used withother load control devices. For example, one or more of the embodimentsdescribed herein may be performed by a variety of load control devicesthat are configured to control of a variety of electrical load types,such as, for example, a LED driver for driving an LED light source(e.g., an LED light engine); a screw-in luminaire including a dimmercircuit and an incandescent or halogen lamp; a screw-in luminaireincluding a ballast and a compact fluorescent lamp; a screw-in luminaireincluding an LED driver and an LED light source; a dimming circuit forcontrolling the intensity of an incandescent lamp, a halogen lamp, anelectronic low-voltage lighting load, a magnetic low-voltage lightingload, or another type of lighting load; an electronic switch,controllable circuit breaker, or other switching device for turningelectrical loads or appliances on and off; a plug-in load controldevice, controllable electrical receptacle, or controllable power stripfor controlling one or more plug-in electrical loads (e.g., coffee pots,space heaters, other home appliances, and the like); a motor controlunit for controlling a motor load (e.g., a ceiling fan or an exhaustfan); a drive unit for controlling a motorized window treatment or aprojection screen; motorized interior or exterior shutters; a thermostatfor a heating and/or cooling system; a temperature control device forcontrolling a heating, ventilation, and air conditioning (HVAC) system;an air conditioner; a compressor; an electric baseboard heatercontroller; a controllable damper; a humidity control unit; adehumidifier; a water heater; a pool pump; a refrigerator; a freezer; atelevision or computer monitor; a power supply; an audio system oramplifier; a generator; an electric charger, such as an electric vehiclecharger; and an alternative energy controller (e.g., a solar, wind, orthermal energy controller). A single control circuit may be coupled toand/or adapted to control multiple types of electrical loads in a loadcontrol system.

What is claimed is: 1-14. (canceled)
 15. A load control device,comprising: a controllable impedance circuit configured to conduct aload current through a light-emitting diode (LED) light source; a firstswitching device connected in series with the controllable impedancecircuit; a feedback circuit configured to generate a feedback signalindicative of a magnitude of the load current conducted through the LEDlight source; a control loop circuit coupled to the feedback circuit andconfigured to generate a drive signal for controlling the controllableimpedance circuit based on the feedback signal, the control loop circuitcomprising a second switching device, the second switching devicecapable of being rendered conductive and non-conductive to control whenthe feedback signal is used to generate the drive signal; and a digitalcontrol circuit configured to control the control loop circuit to adjusta peak magnitude of the load current conducted through the LED lightsource toward a target magnitude, the digital control circuit configuredto render the first switching device conductive and non-conductive via apulse-width modulated (PWM) signal and adjust a duty cycle of the PWMsignal to adjust an average magnitude of the load current, the digitalcontrol circuit further configured to render the second switching deviceconductive and non-conductive in coordination with the PWM signal. 16.The load control device of claim 15, wherein the digital control circuitis configured to render the second switching device conductive at theend of a first time period after the digital control circuit renders thefirst switching device conductive, the digital control circuit furtherconfigured to render the second switching device non-conductive at thebeginning of a second time period before the digital control circuitrenders the first switching device non-conductive.
 17. The load controldevice of claim 15, wherein the digital control circuit is configured torender the second switching device conductive at a first time offsetafter the digital control circuit renders the first switching deviceconductive, the digital control circuit further configured to render thesecond switching device non-conductive a second time offset before thedigital control circuit renders the first switching devicenon-conductive.
 18. The load control device of claim 15, wherein thecontrol loop circuit further comprises a filter circuit configured tofilter the feedback signal and the second switching device is renderedconductive and non-conductive to control when the feedback signal isprovided to the filter circuit.
 19. The load control device of claim 18,wherein the filter circuit comprises a resistor-capacitor (RC) filtercoupled to the second switching device and configured to generate asignal representative of a peak magnitude of the feedback signal whenthe second switching device is rendered conductive.
 20. The load controldevice of claim 15, wherein the control loop circuit further comprisesan integrator circuit, the control loop circuit configured to receive atarget current control signal from the digital control circuit andgenerate the at least one drive signal by integrating the differencebetween the target current control signal and the feedback signal viathe integrator circuit.
 21. The load control device of claim 20, whereinthe control loop circuit is characterized by a time constant, whereinthe load current conducted by the controllable impedance circuit ischaracterized by a load current period, and wherein the time constant ofthe integrator circuit is greater than the load current period.
 22. Theload control device of claim 15, wherein, when the target magnitude isless than a transition value, the digital control circuit is configuredto keep the peak magnitude of the load current conducted by thecontrollable impedance circuit at a constant magnitude and adjust theduty cycle of the PWM signal to adjust the average magnitude of the loadcurrent toward the target magnitude.
 23. The load control device ofclaim 22, wherein, when the target magnitude is greater than or equal tothe transition value, the digital control circuit is configured to keepthe duty cycle of the PWM signal at approximately 99% and adjust thepeak magnitude of the load current conducted by the controllableimpedance circuit to adjust the average magnitude of the load currenttoward the target magnitude.
 24. The load control device of claim 23,wherein, when the target magnitude is greater than or equal to thetransition value, the digital control circuit is configured to keep theduty cycle of the PWM signal at approximately 100% and adjust the peakmagnitude of the load current conducted by the controllable impedancecircuit to adjust the average magnitude of the load current toward thetarget magnitude.
 25. The load control device of claim 15, wherein thefirst switching device is electrically coupled between the controllableimpedance circuit and a circuit common.
 26. The load control device ofclaim 15, wherein the controllable impedance circuit comprises aregulation transistor configured to operate in a linear region.
 27. Theload control device of claim 15, further comprising a bus adjustmentcircuit coupled to the controllable impedance circuit and configured tokeep a voltage developed across the controllable impedance circuit belowa threshold.
 28. The load control device of claim 15, further comprisinga wireless communication circuit, wherein the digital control circuit isconfigured to control the controllable impedance circuit and the firstswitching device in response to a control message received via thewireless communication circuit.
 29. A load control device, comprising: acontrollable impedance circuit configured to conduct a load currentthrough a light-emitting diode (LED) light source; a switching deviceconnected in series with the controllable impedance circuit; a feedbackcircuit configured to generate a feedback signal indicative of amagnitude of the load current conducted through the LED light source;and a control circuit coupled to the feedback circuit and configured togenerate a drive signal for controlling the controllable impedancecircuit based on the feedback signal and control the switching device toadjust a peak magnitude of the load current conducted through the LEDlight source toward a target magnitude, the control circuit furtherconfigured to render the switching device conductive and non-conductivevia a pulse-width modulated (PWM) signal and adjust a duty cycle of thePWM signal to adjust an average magnitude of the load current, thecontrol circuit further configured to control when to sample thefeedback signal in coordination with the PWM signal.
 30. The loadcontrol device of claim 29, wherein the control circuit is configured tosample the feedback signal during a time window, the time windowstarting at the same time or after the control circuit renders theswitching device conductive in each duty cycle of the PWM signal, thetime window ending at the same time or before the control circuitrenders the switching device non-conductive in the duty cycle of the PWMsignal.
 31. The load control device of claim 30, wherein the time windowstarts at the end of a first offset time period after the controlcircuit renders the switching device conductive and wherein the timewindow ends at the beginning of a second offset time period before thecontrol circuit renders the switching device non-conductive.
 32. Theload control device of claim 29, wherein the control circuit is furtherconfigured to filter the feedback signal via a low pass filter andwherein the drive signal is generated based on the filtered feedbacksignal. 33-39. (canceled)